Since a lithium secondary cell has high voltage and high capacity, it has been widely used as an electric source for mobile phones, digital cameras, video cameras, notebook computers, electric cars, etc. The lithium secondary cell which is commonly circulated uses a liquid electrolyte where electrolytic salt as an electrolyte is dissolved in a non-aqueous solvent. Since the non-aqueous solvent contains much combustible solvent, there has been a demand for securing its safety.
Further, in recent years, demand for the lithium secondary cell has been increasing for storing the electric power in generating devices in cars (such as electric car and hybrid car), solar battery, wind power generation, etc. However, since the lithium secondary cell uses lithium wherein the estimated deposit amount is small and the producing places are unevenly distributed, there is an anxiety that the demand exceeds the supply and there is also a problem of high cost.
For solving the above problem, as an all-solid-state secondary cell using no solvent, a sodium-sulfur cell (NAS cell) which is a kind of sodium secondary cell has been receiving public attention as a big-size cell for storage of electric power.
Since the NAS cell is operated at the temperature of as high as not lower than 300° C., careful attention is needed for the handling of sodium in a liquid state and there has been a problem in terms of its safety.
Moreover, in the NAS cell, β-alumina is used as its sodium ion-conductive solid (electrolyte). β-Alumina shows a sodium ion conductivity of not less than 10−3 Scm−1 at room temperature [Document: X. Lu et al., Journal of Power Sources, 195 (2010) 2431-2442]. However, for the production of β-alumina, burning at the temperature of as high as not lower than 1600° C. is necessary and there is a problem that its solid interfacial adhesion to a positive electrode active material is difficult.
Accordingly, in order to provide a highly safe all-solid-state secondary cell, there has been a demand for lowering the operating temperature to about room temperature. In addition, a material which does not need the burning at high temperature and shows high conductivity as a molded product of powder being produced merely by means of press is important for the interfacial constitution comprising a positive electrode and an electrolyte for an all-solid-state secondary cell of a low-temperature operation type.
In view of the above, the inventors of the present invention have proposed to use Na2S—P2S5 glass ceramics as a solid electrolyte (Abstracts of Presentations at the 36th Symposium on Solid State Ionics in Japan, (2010) page 120).